That will be the time I hang up my keys..... Even in the IAM they tell you to go over the speed limit to make an overtake if it is safe to do so, then pull in and reduce your speed.
All you need is somebody doing 30 in a 50 where you have few opportunities to overtake and you'll get a huge line of traffic. An example is the A350, Shaftesbury to Blandford. It's hilly, and twisty, most of it is either 40 or 50 limits, except through the villages, and I have often been sat behind that Honda Jazz doing no more than 35mph anywhere. Where it is safe to overtake, I can accelerate to 70, get past, and come back down to the limit. In fact I've done this before and 2 or 3 cars have followed, so it's not just me that finds these people frustrating...